We will attempt to define more fully the neuroendocrine role of the brain in the control of the corpora allata (CA) during insect larval development. In vivo studies by Granger and Sehnal have suggested for the wax moth, Galleria mellonella, that the CA are activated by a neurohormone ("allatotropin") probably produced by neuorosecretory cells in the pars intercerebralis of the brain. In vitro studies by Williams have indicated that in the tobacco hornworm, Manduca sexta, the decline in juvenile hormone (JH) titer during the last larval instar is in part a result of the interaction with the brain of a humoral inhibition factor which acts by turning off an allatotropin and turning on an allata-inhibiting factor ("allatohibin"). The difficulties encountered in showing a direct effect of neural factors on CA function in vivo and of measuring that effect by the extraction and measurement of JH from the hemolymph underlines the value of the in vitro approach to be used here in analyzing the control of CA function. For quantitation of the biosynthesis of JH by CA in culture, radioimmune assays (RIAs) for JH I and JH III have been adapted to directly measure these JHs in culture medium. With the use of the RIA, optimal culture conditions have been established for the larval CA of manduca, and in vitro titer of CA activity during larval-pupal development is being determined, and the effect of the brain on the biosynthesis of JH by the CA in culture is being assessed. Experiments have been designed so as to demonstrate the existence of an effect of either neurohormones or neurotransmitters on CA function and to differentiate between stimulatory and inhibitory factors. Should the existence of an allatotropin be confirmed by the in vitro studies, we will examine the chemical nature of this neurohormone, hormonal feedback on its synthesis and release, and its activity during larval development.